The present invention relates generally to an optical system and an imaging system incorporating it, and more particularly to a zoom lens well suitable in construction for use on digital or video cameras, in which its optical system portion is so elaborated that albeit having a high zoom ratio, it can be slimmed down with reduced influences of decentration and so little or no performance deteriorations, and an imaging system incorporating the same.
There is now a growing demand for slimming down zoom lenses in general and those for digital cameras in particular. With digital cameras using small-format image pickup devices, in general, lenses may be designed to be smaller than those used with silver halide cameras. However, lens part fabrication errors, and errors upon incorporation of lens parts in a lens barrel become relatively large, often incurring performance deteriorations due to decentration and, hence, resulting in yield drops. Often for slimming down cameras, lenses that form a zoom lens must be as few as possible, and as many aspheric surfaces as possible must be used for correction of aberrations with fewer lenses.
However, such an optical system is susceptible of image-formation capability drops by reason of relative decentration between aspheric lenses due to fabrication errors, and so it is still difficult to offer a sensible tradeoff between high-yield mass fabrication and slimming-down.
Reductions in the number of lenses through use of many aspheric surfaces, for instance, are set forth in patent publications 1, 2, 3 and 4; in any case, however, the sensitivity to decentration of aspheric surfaces in the second lens group is unacceptably high, failing to reduce the total thickness of each lens group to a sufficient level.
There are also patent publications 5, 6 and 7. However, with the zoom lens of patent publication 5 there are problems such as noticeable performance deterioration (regarding off-axis aberrations in particular) ascribable to decentration of fabrication errors of individual lenses, and inadequate correction of chromatic aberrations by reason of a small Abbe constant difference between lenses that forms a cemented lens. The zoom lenses of patent publications 5 and 6 are less than satisfactory in terms of slimming-down, because no aspheric surface is used at all.
Patent Publication 1
JP (A) 1-183616
Patent Publication 2
JP (A) 10-282416
Patent Publication 3
JP (A) 11-95102
Patent Publication 4
JP (A) 11-142734
Patent Publication 5
JP (A) 2004-102211
Patent Publication 6
JP (A) 2002-350726
Patent Publication 7
JP (A) 2004-198855